The present invention relates, in general, to nanofabricated structures, and more particularly to microelectromechanical structures incorporating floating nonvolatile electrostatic charges in relatively movable components.
The growth of integrated heterogeneous technology in recent years has opened many new opportunities for xe2x80x9csystem-on-a-chipxe2x80x9d applications, particularly with the introduction of microelectromechanical systems (MEMS) technology. This technology has produced many interesting applications of sensing and actuation devices and techniques, although the mechanical properties of MEMS structures are limiting. However, many key functions, such as power and communication, are still very difficult to attain with MEMS devices,
For example, one of the mechanical limitations is that the mass of a MEMS device is usually small and not adjustable, so it is difficult to control mechanical harmonics after the device has been fabricated. Therefore, in order to regulate the mechanical properties of a MEMS device, for example to provide mechanical harmonic control for audio/ultrasound microphones and speakers, it has been necessary to design the geometry of the device in such a way as to control stress conditions. Further, it has been necessary to control material density, during the fabrication process, but this cannot be changed after the structure has been produced.
Another difficulty encountered in the past is due to the fact that the sensing of motion in MEMS devices has been done by measuring variations in capacitance caused by the motion, which requires an external power source. Power generation by movable MEMS devices has been achieved only through a voltage elevation produced by induced charges in capacitor plates, which also requires an external voltage source. In the area of communications, the use of MEMS devices as antennas for generating electromagnetic waves has been severely limited by the size of these devices, and by their high-Q inductance.
In addition, movable microstructures in MEMS devices suffer from deterioration caused by friction and consequent wear due to the inevitable physical contact between adjacent relatively movable components. This is a particularly serious problem for applications such as hinges and bearing structures such as might be found in micro-turbine or micro-motor devices. Such physical contact is often due to the attractive forces, such as Van de Walls forces, which may hold very small low mass microstructures together with such force that it is difficult to pry them apart without destroying them, and the resulting frictional wear can severely limit the usefulness and the lifetime of such structures.
Accordingly, there is a need to provide improved MEMS devices and processes which will reduce frictional contact between relatively movable structures to provide friction and wear-free micromechanical structures; which permit active waveform sensing and low-power generator applications without the need for external power; which will provide adaptive and reconfigurable mechanical harmonic control for MEMS devices such as audio or ultrasound microphones and speakers; which will allow the creation of self-contained microsystems; and which will provide a capability for wireless communication.
In accordance with the present invention, problems encountered with prior MEMS devices are overcome through the introduction into MEMS structures of floating nonvolatile electrical charges, such as those available in commercial electrically erasable programmable read-only memory (EEPROM) devices. These floating charges are injected into xe2x80x9cfloating gate structuresxe2x80x9d in MEMS devices, which may be released, movable polysilicon membranes, beams, or other movable or fixed MEMS structures or devices incorporating electrically conductive or semiconductive layers or segments capable of receiving and retaining electrostatic charges. Such structures will hereinafter be referred to as floating gate structures or electrostatically charged membranes or beams. The floating gate structures preferably are embedded in a dielectric such as SIO2, so that charges can be retained over a long period of time, for example, 10 years or more. The electrostatic forces produced by these charges can be much larger (10-106 times larger) than the Newtonian forces which act on microstructures below the dimension of 100 xcexcm. As a result, these electrostatic forces can greatly modify the effective mechanical properties of the MEMS structures or devices to which they are applied, and because they can be externally controlled, the properties of the MEMS device can easily be reconfigured during both the fabrication and operation of the MEMS device.
A preferred method of providing the electrostatic charges in a MEMS device or structure is by way of EEPROM devices having floating gate structures connected to the MEMS structure. The floating gate structure of an EEPROM device is made of polysilicon embedded in insulating dielectrics, and extends outside the EEPROM structure. This exterior floating gate is electrically connected to a moving beam or similar structure of a MEMS device which optionally can be embedded in a very thin passivation dielectric layer. EEPROM devices can be readily provided for in the design and fabrication of MEMS structures, and these devices permit real-time reconfiguration and control of the resulting structures. The injection of floating charges into MEMS structures produces repulsive electrostatic forces between adjacent structures or layers having floating charges of the same polarity, and it has been found that the provision of these forces overcomes the problem of friction and wear in MEMS devices. Thus, for example, repulsive forces can be used to produce friction-free and wear-free microbearings, hinges and turbines whose reliability previously has been limited to 103-106 rotations before failure. For geometrical dimensions below about 100 xcexcm, the electrostatic forces between static charges of the same polarity in MEMS devices are more than strong enough to support the light mass of such structures. For applications where image charges may reduce the effectiveness of static charges, nano-crystal floating gate structures can be employed, and if the ambient gas pressure is lowered by sealing the device in a low pressure environment, the resulting mechanical structure can have extremely low damping characteristics.
The provision of floating charges on movable or vibrating MEMS structures can also revolutionize applications that have previously relied on time-varying capacitance sensing during mechanical movement. For example, in many commerical microphones, vibrating polymer layers have been used to induce waveforms for sensing, but floating charges produced in a MEMS device by an EEPROM can be much stronger in density, easier to program and thus to control, and will last longer than a charged polymer. Further, the vibration of a MEMS structure containing floating charges in accordance with the invention may serve as a transducer to convert mechanical motion to electrical power. This permits a mechanical structure with embedded floating charges to be fabricated on a substrate, or chip, and to be used for on-chip power generation, thereby resolving one of the most serious problems in sub-millimeter autonomous systems. For example, such a transducer may include a floating membrane, which may be highly doped polysilicon layer, located between, and movable with respect to, upper and lower spaced conductive layers. The spaced conductors and the membrane are initially electrically connected to a floating gate of an EEPROM device which is biased to inject electrostatic charges into floating gate structures of the stationary conductors and the floating membrane. Thereafter, motion of the membrane with respect to the conductors induces an AC current in the adjacent conductive layers to produce a usable quantity of electrical power for applications such as biomedical and environmental sensors and actuators, pacemakers, etc.
The static charges embedded in a floating membrane and the adjacent conductors by the EEPROM cell effectively increase the inertia of the membrane and change its mechanical behavior. Thus, for example, the mechanical properties of vibration of a MEMS structure, including its resonant frequency, can be modified by controlling the amount of the floating charge applied to it, as well as by careful design of its geometric layout and control of the fabrication process. These properties can be dynamically reconfigured by controlling the EEPROM which supplies the static charge. Such a device can be used, for example, to detect mechanical vibration of the membrane and thus can be used for mechanical harmonic signature recognition. The sensing membrane motion can be passive, as by inducing an electrical signal waveform in the conductive plates, instead of by measurement of capacitance variations.
The motion of the membrane can also be used to provide vibration-based generation of electromagnetic (EM) waves for wireless telemetry. Such waves, which are dissipated from moving static charges, will be of relatively low power, but will provide near-field and short range EM communication.
When floating charges of the same polarity are injected into both components of a relatively movable mechanical structure, through connection to EEPROM devices, the repulsive force produced by the charges on both components will maintain a floating structure in an equilibrium position with respect to a fixed structure, and will provide protection against contact between the components due to mechanical impact. This significantly reduces wear in the structure and increases its reliability. Such relatively movable structures include mechanical hinge devices, comb-type drive actuators having interdigitated fingers, spring structures for supporting the comb drives, bearings, turbine structures, and the like. Repulsive-force electrostatic charges applied to such mechanical structures maintain separation between adjacent components and overcome stress gradients that can, for example, cause comb-finger tilting and related problems. It has been found that if the separation gap between adjacent structures is larger than about 10 nm, friction which originates from Van de Wall surface forces can be substantially eliminated.
In accordance with the invention, MEMS structures are integrated with, and are electrically connected to, EEPROM devices on a single substrate for efficiency as well as for optimization of the mechanical properties. After static charges have been supplied to the MEMS devices, the EEPROM structures can be etched away, as during the MEMS release step, or the EEPROM structures can be maintained on separate islands but in connection with the mechanical structure to permit dynamic tuning of the floating charge. Alternatively, a charge can be transported to a moving structure from an attached EEPROM, with the metal interconnect being broken by the initial mechanical motion of the structure, leaving the EEPROM and its floating gate intact.